This invention relates generally to gas turbine engines, and specifically to turbine engines for aircraft. In particular, the invention concerns a gas turbine engine with an aft-mounted propulsor assembly.
Turbine engines provide efficient, reliable power for a wide range of industrial applications, including aviation, power generation, and commercial heating and cooling. Gas turbine engines (or combustion turbines) are built around a power core made up of a compressor, combustor and turbine, arranged in flow series with an upstream inlet and downstream exhaust.
The compressor compresses air from the inlet, which is mixed which fuel in the combustor and ignited to generate hot combustion gas. The turbine extracts energy from the expanding combustion gas, and drives the compressor via a common shaft. Energy is delivered in the form of rotational energy in the shaft, reactive thrust from the exhaust, or both.
Large-scale gas turbine engines may include a number of different compressor and turbine sections, which are arranged into coaxially nested spools. The spools operate at different pressures and temperatures, and rotate at different speeds. Individual compressor and turbine sections are subdivided into a number of stages, which are formed of alternating rows of rotor blade and stator vane airfoils. The airfoils are shaped to turn, accelerate and compress the gas flow, and to generate lift for conversion to rotational energy in the turbine.
Traditional aviation applications include turbojet, turbofan, turboprop and turboshaft engines. Turbojet engines are an older design, in which thrust is generated primarily from the exhaust. Modern fixed-wing aircraft typically employ turbofan and turboprop configurations, in which the low spool is coupled to a propulsion fan or propeller. Turboshaft engines are used on helicopters and other rotary-wing aircraft.
Turboprop and turboshaft engines usually include reduction gearboxes to decrease blade tip speeds. The reduction ratio is generally higher for turboshaft engines, due to the larger size of the rotor. Advanced turbofan engines may also include geared drive mechanisms, providing independent fan speed control for increased efficiency and reduced engine noise.
Most commercial and general-purpose military aircraft are powered by two- or three-spool turbine engines, in either a turboprop or high-bypass turbofan configuration. High-bypass turbofans generate most of their thrust via the propulsion fan, which drives airflow through a bypass duct oriented around the engine core. Turboprop engines typically employ open-rotor propeller designs, but ducted propellers and unducted turbofans are also known.
Low-bypass turbofan engines are used on supersonic fighters and other high-performance aircraft. Low-bypass turbofans generate less thrust from the bypass flow and more from the engine core, delivering greater specific thrust but incurring additional costs in noise and fuel efficiency.
As commercial engines trend toward higher bypass designs, engine performance depends on precise control of the fan speed and fan pressure ratio, and the corresponding cost and weight penalties for complex gearing and reduction-drive mechanisms. The problem is particularly evident in the blade tip region, where traditional turbofans and unducted propeller blades operate at transonic and supersonic velocities, resulting in shock wave formation, noise generation, and loss of efficiency.